Il ghiaccio della Groenlandia si sta sciogliendo molto più velocemente del previsto. Scienziati in gara per spiegarne le ragioni
articolo di Kristan Uhlenbrock , 14 August 2015
When viewed from above, the fjords of Greenland look like arteries carrying water and ice from the heart of the mainland ice sheet. At the head of these fjords are some of the world’s largest glaciers. Called marine-terminating glaciers, they constantly recede and advance with the change of the seasons. And every so often, a piece of ice breaks off.
The pieces are never trivial. Imagine a chunk of ice miles across and as tall as a skyscraper, most of it submerged below the water’s surface. The ice crumbles into the ocean, rolling and bobbing around like a rubber duck in a bathtub, and slowly floats out to sea. Ice that used to be part of the glacier now drifts around the ocean as large free-floating icebergs, steadily melting. Occasionally, one of these calving events getscaught on film.
For a long time scientists focused on warming air temperatures as one of the leading causes of the melting of the Greenland ice sheet. Now, researchers have turned their attention to where the ocean and ice meet, typically at the head of fjords that contain these marine-terminating glaciers. By collecting more samples plus using new tools, researchers are shining a light on the complicated underwater picture of how warming Atlantic Ocean waters speed up the melting of the ice, which contributes to sea-level rise.
“Greenland is no longer in balance.”
Many calving events happen in fjords, which were created when glaciers carved out the Earth, cutting rifts with deep sides and narrow inlets that were then filled with the sea as the glaciers retreated.
Greenland’s fjords, like many polar locations, are complicated and tough environments in which to do research. Besides the remoteness and bitter cold conditions, the fjords are full of towering icebergs the size of city blocks. Calving events can send tidal waves heaving out of the inlet and topple a researcher’s boat, and storms blow in quickly.
Straneo in action. Image: Fiamma Straneo/Woods Hole Oceanographic Institution
Fiamma Straneo is not shy about layering up in foul weather gear and driving boats around a fjord, dodging floating icebergs. An oceanographer from Woods Hole Oceanographic Institution in Massachusetts, she has made 11 trips to the region during the summer, 12 if you count her current trip.
Straneo has been studying Greenland and the ocean-ice margin since the late 90s. When starting out, she was hesitant because deploying scientific instruments in these waters was a gamble. Some of her first trips were on an 18-foot skiff owned by the local telephone company. Although she still prefers to go out with locals, her vessel has been upgraded to something similar to a tugboat.
Straneo realized the importance of this region the moment she dropped a sensor a thousand feet below the surface of the fjord and measured the water temperature at 39°F (4°C). This water is surprisingly warm given that the surface waters hover around 32°F (0°C) in July. North Atlantic waters are warming, and ocean circulation patterns and winds are carrying this water into the fjords. Straneo wanted to understand what responsibility the ocean has for the melting of these glaciers. To do so, she needed to know what was going on below the surface.
“Everything is changing. The glaciers have sped up, throwing more icebergs in the ocean, and the surface melt is going through the roof,” said Straneo. “[Greenland] is no longer in balance. It’s leaking ice, which is why we worry about sea-level rise.”
Just recently, a media frenzy transpired when James Hansen and a group of other prominent climate scientists released a discussion paper (before peer review) about how global warming is causing ice sheet instability. In short, they argue that the current projections of sea-level rise are too conservative because they fail to accurately account for how quickly ice sheets are likely to melt.
In 2012, the Greenland ice sheet lost more ice than in any other year in the last three decades. In 2014, the recorded loss of ice wasn’t far behind, ranking 7th according to the US National Snow and Ice Data Center. In 2015, the ice sheet is melting at a rate well above the 1981-2010 average.
Sailing in Greenland isn’t for the faint of heart. Image: Fiamma Straneo/Woods Hole Oceanographic Institution
The Greenland ice sheet is shedding ice at its edges and the rate of ice loss hasquadrupled over the past two decades. As the globe continues to warm, the melting of the Greenland ice sheet is anticipated to increase and be a major contributor to sea-level rise. How fast this will happen is still uncertain, as is how much of the ice sheet will actually melt. Scientists also don’t yet understand the complex dynamics that drive this melting. But they are getting closer.
New underwater data shows warm Atlantic water reaching far up into the fjords and cutting out the undersides of glaciers, causing them to melt faster and become unstable, according to a recent publication from a team led by Eric Rignot of the University of California-Irvine and NASA Jet Propulsion Laboratory. The study found that the undercutting of these marine-terminating glaciers is intensifying both the receding of the glaciers and the calving events. “It’s like putting salt on ice,” said Rignot during a phone interview. “It changes the melting point of ice and makes it melt much faster.”
He stressed that the ocean is melting the glacier on the order of meters per day, which is significantly more substantial than the role of air temperatures melting it a few centimeters per day. Yet future projections of sea level rise don’t accurately capture the ocean’s influence on the melting of these glaciers. “Existing numerical models have no clue about this plumbing,” said Rignot. “In effect, we find that the glaciers are far more vulnerable to climate warming than represented in any model.”
Another piece of the already complicated puzzle is how melting on the top of the ice sheet can speed up the process. As Greenland’s surface ice melts, the water drains into cracks and goes all the way to the bedrock, and eventually escapes into the same fjords where the glaciers run into the ocean.
“It’s like turning a spigot on at the base of the glacier in the fjord, maybe 600 meters [2000 feet] below sea level,” Straneo said.All this freshwater entering the sea causes major turbulence near the ice, which draws in warmer Atlantic waters and enhances melting. She says that the warmer water probably triggers calving events too. These dynamics are so difficult to model because they only happen at the edge of the glaciers.
Calving is part of a glacier’s natural cycle, but scientists think the rate of these events is increasing as a result of climate change. James Balog, founder and director of theExtreme Ice Survey and subject of the documentary Chasing Ice, has spent years observing these extreme environments and witnessing glaciers calve.
“It shakes one’s sense of reality,” he said. “You are seeing something that is so far outside the realm of a normal experience that even though it turns out to be an acutely embedded memory in the brain that you never forget—there is another part of the brain that goes, ‘Did that really just happen?’”
“One of the mind’s reflexes is to go into doubting mode, despite of the fact your eyes just saw it. Your feet were on the ground experiencing it,” said Balog. “It’s like watching a car crash in front of you in the road.”
However, the risk of working in these harsh environments doesn’t stop scientists who know collecting these data are important if we want to better predict future climate. “We didn’t have the data in the past to answer the questions when the glacier startedto change, so if we don’t keep collecting it, we won’t have it for future generations,” said Straneo. “The next generation will be in the same spot that we are, if we don’t continue to collect data.”
The stability of the Greenland ice sheet is under pressure from a warming wold, both on top of the ice and also underwater where research has been limited. Today, Straneo is currently in Greenland collecting and redeploying sensors to study how the ocean interacts with glaciers and how the system is changing. By looking below the surface, she and other scientists are better equipped to predict the future threat of sea-level rise, and it appears the ocean may be the biggest force of all.
Hell or Salt Water is a series on Motherboard about exploring and preserving our oceans. Follow along here.